Transceive surface coil array for magnetic resonance imaging and spectroscopy

ABSTRACT

A surface coil array comprises a surface coil support and an arrangement of non-overlapping magnetically decoupled surface coils mounted on the support. The surface coils encompass a volume into which a target to be imaged is placed. Magnetic decoupling circuits act between adjacent surface coils. Impedance matching circuitry couples the surface coils to conventional transmit and receive components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/554,350 filed on Mar. 19, 2004 for an inventionentitled “Transceive Surface Coil Array For Magnetic Resonance Imagingand Spectroscopy”.

FIELD OF THE INVENTION

The present invention relates generally to magnetic resonance imaging(MRI) and more specifically, to a transceive surface coil array formagnetic resonance imaging and spectroscopy.

BACKGROUND OF THE INVENTION

Nuclear Magnetic Resonance (NMR) Imaging, or Magnetic Resonance Imaging(MRI) as it is commonly known, is a non-invasive imaging modality thatcan produce high resolution, high contrast images of the interior of thehuman body. MRI involves the interrogation of the nuclear magneticmoments of a subject placed in a strong magnetic field with radiofrequency (RF) magnetic fields. A MRI system typically comprises a fixedmagnet to create the main strong magnetic field, a gradient coilassembly to permit spatial encoding of signal information, a variety ofRF resonators or RF coils as they are commonly known, to transmit RFenergy to, and receive signals emanating back from, the subject beingimaged, and a computer to control overall MRI system operation andcreate images from the signal information obtained.

The design of RF resonators operating in the near-field regime plays amajor role in the quality of magnetic resonance imaging. Over the pastfew decades, the designs of RF resonators have significantly evolvedfrom the simple solenoid coils of wire that were typically used. Thevolume coil, such as for example the saddle coil, hybrid birdcage orTEM, has been a popular choice of MRI transmitter and/or receiver forlarge fields of view (FOV) due to the ability to match it to 50Ωimpedance transmit and receive components and its homogeneoussensitivity profile. Although these volume coils operate well at lowmagnetic field strengths, they become less effective in the high fieldregime i.e. at magnetic field strengths greater than 3T. As the staticmagnetic field strength used in MRI increases, the wavelength of theassociated Larmor RF approaches the dimensions of the volume coil andvolume of interest (VOI). Several imaging problems arise in this fullwavelength regime namely, increased radiation losses, increased localand global specific absorption rate (SAR), and dielectric resonanceeffects that create both inhomogeneous images and signal loss.

By using surface coils to receive, one can reduce the detrimentaleffects of dielectric resonance on signal homogeneity commonly observedwith volume coils at higher field strengths (>3 T). Surface coils canalso be designed to target specific VOIs thereby to reduce unnecessarypower deposition within the patient. Furthermore, the increasedsensitivity of surface coils for reception, in comparison to volumecoils, presents an opportunity for increased image signal-to-noise ratio(SNR) in both receive-only and transmit and/or receive (“transceive”)modes.

With the advent of fast parallel imaging techniques such as SMASH, SENSEand transmit SENSE, there exists a greater need for flexible placementand combination of multiple receiver and/or transmitter RF coils. Thesensitivity profiles of these multiple RF coils are required for andinfluence the efficiency of these fast parallel imaging methods. Fastparallel imaging techniques provide the ability to remove or unfoldaliasing artefacts in under-sampled images. This ability to un-aliasimages provides a means to increase temporal resolution. Images that mayhave been impossible to acquire within the time constraints of breathhold techniques may be realizable through the reduction of motionartifacts.

There are several design approaches for both volume and surface coilsthat have been implemented to acquire multiple sensitivity profiles withconsiderable success. For example, the degenerate mode birdcage has beenshown to be useful in sensitivity encoding although receive-only surfacecoil arrays provide higher SNR and are more suitable for high field MRI.The predominant impediment to surface coil array design is however, thestrong magnetic coil-to-coil coupling.

There are several design approaches available to reduce this magneticcoil-to-coil coupling, including preamplifier decoupling, striptransmission line arrays, overlap geometries, and capacitive decouplingnetworks

Magnetic coil-to-coil coupling can be substantially eliminated betweentwo (2) neighboring surface coils using a unique overlap of surfacecoils. Unfortunately, the resultant overlapping sensitivity profiles areless than ideal for fast parallel imaging techniques, which are moreeffective when sensitivity profiles of individual surface coils do notoverlap.

Alternatively, the effect of magnetic coil-to-coil coupling on bothnearest neighbor and next nearest neighbor surface coils can be reducedusing a decoupling method employing low impedance preamplifiers.Unfortunately, low (or high) impedance preamplifiers are not generallyavailable off-the-shelf and are therefore, more complex, expensive andtime consuming to implement. Furthermore, low input impedance RF poweramplifiers are not widely commercially available off-the-shelf, therebypractically limiting this coil-to-coil magnetic coupling reductiontechnique to receive-only applications. More complicated capacitiveladder networks have been employed to reduce magnetic coil-to-coilcoupling at lower field strengths. However, considerable electric fieldloss and strong coupling between lattice networks limits theirapplication at higher field strengths.

Other coil-to-coil coupling reduction techniques have also beenconsidered. For example, U.S. Pat. No. 5,973,495 to Mansfield disclosesa method and apparatus for eliminating mutual inductance effects inresonant coil assemblies in which a plurality of coils is situated insufficiently close proximity to create small mutual inductances betweenthe coils. Mutual inductances are evaluated using a T star or othertransformation of the relevant parts of the circuit thereby to isolatethe inductances in such a way that series capacitances may be introducedto tune out the mutual inductances at a common frequency, reducing thecoil array to a synchronously tuned circuit. Unfortunately, this designrequires a common ground resulting in electric field losses and requiresa common connection between all of the coils, which is geometricallyrestrictive.

U.S. Pat. No. 6,788,059 to Lee et al. discloses an RF detector arraybased on a microstrip array decoupling scheme. The detector arraycomprises a plurality of conductive array elements that is substantiallyparallel to a conductive ground plane and a plurality of capacitors. Atleast one capacitor is shunted from each conductive array element to theground plane to adjust a corresponding electrical length of eachconductive array element. A combination of each respective conductivearray element, at least one corresponding capacitor and the ground planeforms a resonator that resonates at a selected frequency. A decouplinginterface and a plurality of matching boxes match each decoupled stripto a selected impedance.

U.S. Patent Application Publication No. 2002/0169374 to Jevtic disclosesa capacitive ladder network to achieve next nearest neighbor (NNN)coil-to-coil decoupling. Unfortunately, this ladder network is complexand appears to be limited to low field MRI applications, as considerableelectric field losses, and strong coupling between lattice networkswould limit its application at high field strengths.

U.S. Patent Application Publication No. 2003/0184293 to Boskamp et al.discloses a multiple channel array coil for magnetic resonance imaging,that similar to Lee et al., is based on a microstrip array decouplingscheme. The array coil includes a plurality of conductive strips formedwithin a dielectric medium. The conductive strips are arranged into agenerally cylindrical configuration with each of the strips having alength selected to cause each of the conductive strips to serve as aresonator at a frequency corresponding to a proton MRI frequency. Thecylindrical configuration of the conductive strips forms a multiplechannel, volume resonator in which each of the conductive strips isisolated from the remaining strips.

As will be appreciated, there exists a need for a surface coil arraythat is capable of transmit and/or receive operation for use in fastparallel imaging techniques such as SENSE imaging. There is a furtherneed for a surface coil array that is capable of operation at both lowand high magnetic field strengths without succumbing to SAR limitations.There is a further need for a surface coil array that can operate withconventional 50Ω transmit and receive components including preamplifiersand less expensive low power amplifiers, while maintaining the SNRbenefits of receive-only surface coils.

It is therefore an object of the present invention to provide a novelsurface coil array for magnetic resonance imaging and spectroscopy.

SUMMARY OF THE INVENTION

Accordingly, in one aspect there is provided a surface coil arraycomprising:

a plurality of spaced surface coils arranged about a volume; and

magnetic decoupling circuits acting between said surface coils.

In one embodiment, the magnetic decoupling circuits act between adjacentpairs of surface coils. The magnetic decoupling circuits are purelycapacitive with each magnetic decoupling circuit including a singlecapacitor. The capacitive reactance of each magnetic decoupling circuitis equal to the mutual inductive reactance between adjacent surfacecoils.

The surface coils are generally evenly spaced about the volume and aremounted on a dielectric support. Dielectric loss reducing elements inthe form of low-loss lumped capacitors are distributed about the surfacecoils.

Baluns are coupled to the surface coils and are impedance matched tofeed network circuitry coupled thereto. The feed circuitry is 50Ωcomponent-based. The baluns in one embodiment are configured to providetransmit signals generated by the feed network circuitry to the surfacecoils for transmit operation and to provide receive signals received bythe surface coils to the feed network for receive operation.Alternatively, in another embodiment the baluns are configured tocondition the surface coil array to a receive-only mode. In thereceive-only mode, the baluns isolate the surface coils from the feednetwork circuitry when the feed network circuitry generates transmitsignals.

According to another aspect there is provided a surface coil arraycomprising:

a surface coil support;

a plurality of generally evenly spaced surface coils mounted on saidsupport and surrounding a volume into which a target to be imaged isplaced; and

capacitive decoupling circuitry acting between said surface coils toreduce magnetic coil-to-coil coupling.

According to yet another aspect there is provided an RF resonatorcomprising:

a surface coil array including an arrangement of non-overlappingmagnetically decoupled surface coils encompassing a volume; and

a feed network coupled to said surface coil array, said feed network atleast receiving signals received by said surface coils during imaging ofa target within said volume.

According to still yet another aspect there is provided a surface coilarray comprising:

a surface coil support; and

an arrangement of non-overlapping magnetically decoupled surface coilsmounted on said support and encompassing a volume into which a target tobe imaged is placed.

The surface coil array is advantageous in that it provides theflexibility of using a single array for transmitting and/or receiving RFsignals, which has benefits in fast parallel imaging techniques andlimits image artefacts associated with using separate transmit andreceive coil arrays. The surface coil array is also less SAR limited athigh field strengths due to the proximity of the surface coil array tothe imaging volume, thereby limiting electric field losses. Further, thesurface coil array can be tuned for a variety of paramagnetic nuclei(e.g. ¹³C, ¹H, ²³Na, ³¹P, etc. . . . ) for use in many MR imaging andspectroscopy applications.

The surface coil array provides for the ability to vary surface coilsize and geometry thereby offering great flexibility in custom imagingapplications (e.g. whole body imaging (TIM)). As the surface coil arraycan be used with multiple, more economical lower power amplifiers fortransmit applications, more control in imaging sequences is available.Also, the surface coil array provides for use with conventional 50Ωtransmit and receive components while maintaining isolation betweensurface coils.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described more fully with reference to theaccompanying drawings in which:

FIG. 1 is a perspective view of a transceive surface coil array formagnetic resonance imaging and spectroscopy and an associated feednetwork;

FIG. 2 is an equivalent circuit diagram for the transceive surface coilarray in a transceive mode;

FIG. 3 is an equivalent circuit diagram for the transceive surface coilarray in a receive-only mode.

FIG. 4 shows the spectral profile for each surface coil of thetransceive surface coil array;

FIG. 5 shows the spectral profile for an isolated and unisolated surfacecoil;

FIG. 6 is a graph showing isolation between surface coils as function ofsurface coil separation;

FIG. 7 shows field profiles of a cylindrical oil phantom generated usingthe transceive surface coil array;

FIG. 8 are SNR maps within a human generated using the transceivesurface coil array and using a hybrid birdcage;

FIG. 9 shows transmit fields and contour plots for the transceivesurface coil array in the transceive mode and for a hybrid birdcage; and

FIG. 10 shows SNR images and smoothed contour plots of a human headcaptured using the transceive surface coil array in the transceive mode,the transceive surface coil array in the receive-only mode, a hybridbirdcage and an oversized head coil.

DETAILED DESCRIPTION OF THE EMBODIMENT

Turning now to FIGS. 1 and 2, a transceive surface coil (TSC) array formagnetic resonance imaging and spectroscopy is shown and is generallyidentified by reference numeral 20. As can be seen, TSC array 20comprises a cylindrical supporting shell 22 on which are mounted aplurality of generally equally spaced, rectangular surface coils 24. Theshell 22 is formed of acrylic and has inside and outside diameters ofapproximately 24.1 cm and 25.4 cm respectively. In this particularexample, the TSC array includes eight (8) surface coils 24. Each of thesurface coils 24 is substantially identical and is constructed ofconductive strips, in this case copper tape, having a thickness equal toapproximately 50 μm. Each surface coil 24 includes two legs 26 having alength of approximately 21 cm and two legs 28 having a length ofapproximately 8.1 cm. The width of each leg is approximately 63.5 mm andthe inter-surface coil spacing is approximately 2 cm.

A magnetic decoupling circuit (MDC) 30 extends between each adjacentpair of surface coils 24. The magnetic decoupling circuits 30 are purelycapacitive with each MDC including a single capacitor having acapacitance C_(m). The capacitive reactance (1/(ωC_(m))) is equal to themutual inductive reactance (|ωM|) between each surface coil at 170.3MHz. The value of C_(m) is function of the inter-surface coil spacing aswill be described.

Each surface coil 24 is coupled to a feed network 38 via a quarterwavelength (λ/4) lattice balun 40 and a 50Ω coaxial cable 42. Theinductor and capacitor arrangement of the λ/4 lattice baluns 40 is shownin FIG. 2. Low-loss lumped capacitors 44 are also distributed about thesurface coils 24 to reduce dielectric losses (known as irrotational orcoulomb electric field losses at high magnet fields) within the volumeof interest of the TSC array 20.

The feed network 38 in the present example includes conventional 50Ωtransmit and receive components including 50Ω preamplifiers and lowcost, low power RF amplifiers. The feed network 38 splits a singletransmit signal into eight (8) equally power divided signals (−9 dB)with a phase delay of 2π/8 between each signal and applies the signalsto the surface coils 24 via the coaxial cables 42 and λ/4 lattice baluns40 to approximate current distributions of a birdcage coil operating ina homogenous mode. The feed network 38 also recombines signals receivedby the surface coils 24 into four (4) channels via four (4) 90° branchline couplers and provides the received signals on coaxial cables 46.

The capacitors 30 act as magnetic decoupling circuits between nearestneighbor surface coils 24 that substantially eliminate the effects ofnearest neighbor mutual inductance on resonance splitting and signal andnoise correlation. The capacitance C_(m) has a unique relationship withthe magnitude of the mutual inductance (M) given by:

$\begin{matrix}{C_{m} = \frac{1}{\left. \omega^{2} \middle| M \right|}} & \lbrack 1\rbrack\end{matrix}$

It should be noted that even though k_(m) can be both positive andnegative, the decoupling reactance remains capacitive and may remain inthe same circuit layout. Strong and weak magnetic coupling betweensurface coils 24 can be substantially eliminated irrespective of theinter-surface coil spacing by adjusting the capacitance C_(m) of thecapacitors 30.

Although non-next nearest neighbor coupling may introduce peak splittinginto the spectrum, loading due to both patient conductivity andimpedance matching to the 50Ω coaxial cable has the effect of de-Qingthe surface coils 24 thereby to provide spectral smoothing. Thisspectral smoothing effectively reunifies numerous peaks caused bynon-next nearest neighbor interactions. For cylindrical TSC arraygeometries commonly used in head imaging and in this example, thenon-next nearest neighbor coupling is sufficiently weak that the act ofimpedance matching to the 50Ω coaxial cables 42 unifies any resonancesplitting.

The capacitive reactance (X_(C)) required to eliminate resonance peaksplitting at each separation is equal and opposite to the reactance ofthe mutual inductance (X_(M)), (L_(ind)=260 nH) [cross correlationbetween X_(Cm) & X_(M)=−96.5%, p=1.6E-6, confidence interval a=0.95,N=11]. This magnetic decoupling reactance (X_(Cm)) effectively shortsthe positive feedback coupling between resonance modes into degeneracy.As a result, the magnetic decoupling circuits 30 are able to decoupleboth strong and weak magnetic coupling. In the extreme case of k_(m)=1,the required decoupling capacitance (3.3 pF) is physically realizable.As the mutual inductance approaches zero, the capacitance required fordecoupling approaches infinity. Fortunately, the magnetic decouplingcircuits 30 become redundant when there is no mutual inductance betweenthe surface coils 24.

Closed loop current paths in close proximity to surface coils 24 at highfields couple and degrade spectral responses, analogous to the effectsof neighbouring surface coils within the TSC array 20. The benefits ofusing λ/4 lattice baluns 40 to reduce conservative electric fieldgenerated noise outweigh the detriments due to coupling. To avoid theadditional closed loop current paths introduced by matching networks,the λ/4 lattice baluns 40 are used to both balance and match the surfacecoils 24 to the 50Ω coaxial cables 42. Furthermore, the parallel toseries capacitance ratio may be used to reduce the effects of thevarying patient to patient to loading on the overall matching. For thetransceive mode, an open parallel circuit matched and tuned design has aspectral profile less influenced by other surface coils although it ismore susceptible to impedance mismatch in patient-to-patient variance.Without proper balancing, the transmission line shield voltage potentialrises above ground potential due to direct conductive coupling, and theshield acts as an antenna and increases the amount of conservativeelectric field coupling to the sample and noise correlation detected inthe system.

The TSC array 20 can also be conditioned to a receive-only mode byaltering the λ/4 lattice baluns 40. As shown in FIG. 3, in thereceive-only mode, the λ/4 lattice baluns 40 include PIN diodes 60. ThePIN diodes 60 electrically open each of the surface coils 24 duringtransmit operation of feed network 38.

To test the TSC array 20, a number of images were captured. All imagingdata were obtained using 4 T Varian Unity INOVA whole-body MRI/MRSsystem (Palo Alto, Calif., USA) interfaced to Siemens Sonata Gradientsand amplifiers (Erlangen, Germany).

In a first series of tests, the MR images were obtained using a spoiledsteady state free precession imaging sequence, TR=22 ms, TE=15 ms,NRO=256 NPE=256, sweep width 60 kHz, FOV 20×20 cm, Δz=1 cm. The SNR fromthe both a hybrid birdcage and the TSC array 20 was calculated accordingto modified sum of squares method proposed by Constantinides et al. inthe article entitled “Signal to noise measurements in magnitudes fromNMR Phase Array” published in Magnetic Resonance Medicine Vol. 38, pages852 to 857 (1997) to account for the noise amplification of magnitudeimages when combining data from multiple receivers. The four (4)receiver configuration of the TSC array 20 restricted use of all eight(8) channels independently. The feed network 38 equally transmittedpower to all eight surface coils 45° out of phase and recombined coilsin quadrature pairs to conform to the four (4) receiver configuration.

The gradient coupling between the non-shielded TSC array 20 and systemgradients was not compensated for. A resonance frequency of 171.2 MHzwas measured post acquisition and would suggest that better SNR isattainable.

Two key parameters to assess the TSC array 20 are the spectrum profile,and the isolation between neighboring surface coils 24. Provided thenon-next nearest neighbor interactions are weak, or there is sufficientloading, the array of surface coils 24 act as they would independently.A shielded field probe was centered on the inside of the transmit coilwith all other coils loaded with 50Ω, and a loader shell to mimic thehuman head. As shown in FIG. 4, the spectral profile of each individualsurface coil 24 is identical with the spectral response of that of anisolated surface coil as shown in FIG. 5, which validates theeffectiveness of the magnetic decoupling scheme. The average isolationbetween surface coils 24 as a function of separation, measured with aHewlett-Packard 4395A network/spectrum/impedance analyzer, is shown inFIG. 6 with respect to a through calibration. As expected, there iscylindrical symmetry of isolation with respect to surface coilseparation. Since arbitrary orientation was chosen to measureseparation, symmetry about 180° is expected. The maximum couplingbetween surface coil pairs is approximately −25 dB.

A centered uniform cylindrical oil phantom having a diameter equal to12.7 cm was used to test the recombination of orthogonal surface coils24 through the feed network 38. An annulus loader shell (13 g NaCl, 1.9g CuSO₄, 5.9235 L, inner diameter 16.5 cm, outer diameter 22.9 cm) wasused to load the TSC array 20 to 50Ω. The field profiles of the fourrecombined receive channels are shown in FIG. 7. Each of the fourreceivers can be seen to be a superposition of orthogonal surface coilsexemplifying the aforementioned high isolation between the receivers.

SNR maps as shown in FIG. 8 were used to compare the TSC array 20 to ahybrid birdcage. The TSC array showed a 38% increase in average SNRmeasured throughout the entire head volume. A 9 fold increase in SNR wasobserved when the patient was in close proximity to the TSC array 20. Inthe center of the brain there is an 8% decrease in SNR (centered 11×11voxel ROI).

In a second series of tests, the MR images were obtained using a spoiledsteady state free precession (FLASH) sequence. The |β⁺| profilemeasurements were made by implementing the method proposed by Wang etal. in the article entitled “Measurement and correction of transmitterand receiver induced non-uniformaties in vivo” published in MagneticResonance Medicine, Vol. 53, pages 408 to 417 (2005) that uses thesignal intensities of 2 spin echo sequences (TR/TE 5000/13 ms,FOVx=FOVy=25 cm, NRO=NPE=64) with exciting/refocusing flip angles of60°/120° and 120/240° respectively. The imaging parameters used for allhead SNR images were: FOVx=FOVy=24 cm, TR=20 ms, TE=5 ms, a slicethickness of 1 cm, tip angle of 11°, NRO=NPE=256, with 2 averages perimage. SNR maps were calculated for all surface coils 24 using themagnitude NMR phased array method of Constantinides et al. referencedabove to account for additional magnitude noise accumulating fromcombination of multiple receiver channels. All isolation and spectralprofiles (with the use of shielded RF field probes) measurements weremade with a Hewlett-Packard 4395A network/spectrum/impedance analyzer.

The capacitive decoupling circuits 30 achieve approximately −25 dB ofisolation (or better) between surface coils 24. In comparison, a typicalisolation between a surface coil pair of similar geometry decoupledthrough natural decoupling (10% overlap) at 170.3 MHz is approximately−20 dB. This high degree of isolation between all surface coils 24within the TSC array 20 allows each surface coil to act as it would inisolation. Hence, each surface coil 24 transmits and receivesindependently of each other. The RF spectral profiles as measured withfield probes of each surface coil 24 within the TSC array 20 (loadedwith an annular loader shell) do not differ from the spectral profile ofa single surface coil in complete isolation, confirming the independenceof each surface coil within the TSC array 20. Upon biasing inreceive-only mode, the active decoupling of the TSC array providesgreater than −20 dB of isolation relative to its unbiased field probemeasurement. As a result, the transmit power for the oversized head (OH)coil required to achieve a 90° in both the transceive mode and thereceive-only mode (using the TSC array modified for receive-onlyoperation and the OH coil to transmit) are identical.

A limitation of a surface coil for volume imaging is the penetrationdepth that it can achieve. To assess SNR performance, measurements weremade within the human head of a volunteer. The transmit field |β⁺|profiles for the TSC array 20 in the transceive mode and a hybridbirdcage are shown in FIG. 9. The maximum (mean) transmit fieldinhomogeneity (relative to each surface coil's maximum field intensity)for the TSC array and hybrid birdcage are 4.2% (97.0) and 1.2% (99.3%)respectively. SNR images and smoothed contour plots of a human headcaptured using the TSC array 20 in the transceive mode (TR), the TSCarray 20 in the receive-only mode (RO), a hybrid birdcage (BC) and anoversized head coil (OH) are shown in FIG. 10. These SNR images werereconstructed using the phased array reconstruction method proposed byConstantinides et al. referred to above. The mean SNR values within thebrain volume and at the center of the brain (50×50 pixel region ofinterest (ROI) as outlined by the white box in part (d) of FIG. 10 areshown in Table 1 below:

TABLE 1 Coil Type TR RO BC OH Mean SNR 57.3 48.1 41.6 23.8 Center SNR48.1 41.9 53.9 28.6Compared to the standard hybrid birdcage a decrease of 10.7% (22.3%) inSNR is measured in the center brain region and an increase of 37.7%(15.6%) in SNR over the whole brain volume for the TSC array 20 in thetransceive (receive-only) mode. A maximum 9-fold increase in SNR ismeasured in the brain region using the TSC array 20 as compared to thehybrid birdcage. There exists a 10.7% decrease of SNR in the centerbrain region of the TSC array 20 in the transcieve mode compared to thehybrid birdcage. Both hybrid birdcage values demonstrate a pronounceddielectric resonance in the centre of the array, with the centre SNRconsiderably enhanced relative to the mean.

The SNR of the TSC array 20 in the transceive mode is slightly higherthan in the receive-only mode (see Table 1 and FIG. 10). Since thereceive profiles are identical for both modes of operation (through theuse of active decoupling), it would suggest that the TSC array 20 hasbetter transmit capabilities. In practice, it is common to use a largertransmit RF coil than the receiver (when in receive-only mode) since twocoils cannot occupy the same space within the magnet. The transmitcapabilities of the TSC array 20 (when driven properly) areapproximately equivalent to that of a comparable size hybrid birdcage.The SNR increase of the TSC array 20 in the transceive mode as comparedto the TSC array operating in the receive-only mode could be attributedto better phase performance than the larger oversized head coil,resulting in effectively better quadrature than the larger coil.Although active decoupling provides a high degree of isolation, it ispossible that the copper tape used to form the surface coils 24 shieldedthe sample from approximately 25% of the transmitted RF from theoversized head coil. The recent trend towards prolific use of multiplesurface coils to increase fast parallel imaging efficiency would makethis detrimental shielding effect more pronounced in the receive-onlymode, making the transceive mode more desirable for higher SNR.

The 9-fold increase of SNR at the periphery of the brain with the TSCarray 20 as compared to the hybrid birdcage can be attributed to thesuperior SNR sensitivity of the surface coils 24 in close proximity tothe VOI (as seen in FIG. 10). This increase in peripheral SNR iscomparable to results previously cited in the literature for surfacecoil arrays (SNR increase of six (6) in the human head for areceive-only surface coil array as proposed by Bodurka et al. in thearticle entitled “Scalable Multichannel MRI Data Acquisition System”published in Magnetic Resonance Medicine, Vol. 51, pages 165 to 171(2004)). A notable decrease in SNR is shown in the deep brain comparedto the twofold increase shown by Bodurka et al. Dielectric resonance ofthe human head at 4T has a “bulls-eye” profile, which effectivelydecreases the sensitivity of the birdcage resonator in the peripheralregions of the brain and increases its sensitivity in the center of thebrain. This apparent increase of signal for birdcage or TEM coils in themiddle of the brain may explain the decrease (11%) in deep brain SNRmeasured for the TSC array 20, which inherently displays signal fall offdeep into the VOI and which therefore may not have such strikingdielectric effects. These results suggest that a conformal surface coilarray may find application at very high fields, e.g. 7T, wheredielectric resonance effects dominate.

The variability of subject loading at each surface coil within the TSCarray results in a correspondingly variable transmit power required fora 90° tip. A solution to this problem would be to use multiple low powerRF amplifiers to transmit to each surface coil independently, or toimplement the design on an elliptically conformal coil former to provideequal loading of all surface coils. This first solution would providesthe ability to individually tailor the B1 field (both magnitude andphase) of each surface coil allowing for a means to achieve transmitSENSE and a means to accurately calibrate the transmit power deliveredto each surface coil while being less costly than a single high power RFamplifier.

Through the use of capacitive decoupling circuits and surface coilplacement, the TSC array 20 is able to transmit and receive through eachsurface coil independently while maintaining the use of conventional 50Qamplifiers and preamplifiers. The high SNR of receive-only surface coilsand fast parallel imaging capability can be achieved with a singlemultiple surface coil array, which can easily be incorporated intoexisting MR systems. In addition to proton applications, the TSC arraycould also find use in X-nucleus applications, where a homogeneous bodycoil for transmitting is not available.

Although the TSC array has been described as including eight (8) surfacecoils, those of skill in the art will appreciate that the TSC array mayinclude more or fewer surface coils. Also, the surface coils need not bemounted on a cylindrical supporting shell. Other surface coil supportconfigurations can be used depending on the particular target to beimaged. Further, the surface coils need not be rectangular. The shapeand sizes of the surface coils can be tailored to the particular imagingenvironment in which the TSC array is being used. In addition, whilemagnetic decoupling circuits are shown interconnecting each adjacentpair of surface coils, those of skill in the art will appreciate thatdifferent magnetic decoupling circuit configurations can be used. Forexample, separate sets of surface coils can be magnetically decoupledindependently of one another. In particular, in the case of a TSC arrayconfigured to image a patient's prostate, the TSC array includes twoseparate four (4) surface coil sets, each set of which is magneticallydecoupled independently.

Although a single feed network is shown, those of skill in the art willappreciate that a separate circuit may be used to provide each surfacecoil with a transmit signal. This reduces the power required on a persurface coil basis enabling the use of multiple lower power RFtransmitters and enabling transmit SENSE capabilities. In addition, thesignal received by each surface coil can be applied to downstreamcircuitry independently of the feed network through TR switched or othertechnology enabling receive parallel imaging such as SENSE or SMASH.

Although preferred embodiments have been described, those of skill inthe art will appreciate that variations and modifications may be madewithout departing from the spirit and scope thereof as defined by theappended claims.

1. A transceive surface coil array comprising: a plurality of spacedsurface coils arranged about a volume, each surface coil transmittinginput transmit signals and receiving incoming receive signals; andmagnetic decoupling circuits acting between and magnetically decouplingsaid surface coils, said magnetic decoupling circuits being electricallyconnected in parallel with said surface coils.
 2. A transceive surfacecoil array according to claim 1 wherein magnetic decoupling circuits actbetween adjacent surface coils.
 3. A transceive surface coil arrayaccording to claim 2 wherein said magnetic decoupling circuits arepurely capacitive.
 4. A transceive surface coil array according to claim3 wherein the capacitive reactance of each magnetic decoupling circuitis equal to the mutual inductive reactance between adjacent surfacecoils.
 5. A transceive surface coil array according to claim 4 whereineach magnetic decoupling circuit comprises a single capacitor.
 6. Atransceive surface coil array according to claim 4 wherein said surfacecoils are generally equally spaced about said volume.
 7. A transceivesurface coil array according to claim 5 further comprising a dielectricsupport on which said surface coils are mounted.
 8. A transceive surfacecoil array according to claim 7 further comprising dielectric lossreducing elements distributed about said surface coils.
 9. A transceivesurface coil array according to claim 8 wherein said dielectric lossreducing elements are low-loss lumped capacitors.
 10. A transceivesurface coil array according to claim 4 further comprising balunscoupled to said surface coils.
 11. A transceive surface coil arrayaccording to claim 10 wherein said baluns are impedance matched to feednetwork circuitry coupled thereto.
 12. A transceive surface coil arrayaccording to claim 11 wherein said baluns are impedance matched to 50Ωcomponent-based feed network circuitry.
 13. A transceive surface coilarray according to claim 10 further comprising a feed network coupled tosaid baluns and communicating with said surface coils.
 14. A transceivesurface coil array according to claim 13 wherein said baluns arebalanced and impedance matched to said feed network.
 15. A transceivesurface coil array according to claim 14 wherein said feed networkcomprises transmit and receive components.
 16. A transceive surface coilarray according to claim 15 wherein said transmit and receive componentscomprise 50Ω preamplifiers and low cost radio frequency (RF) poweramplifiers.
 17. A transceive surface coil array according to claim 16wherein said feed network further comprises a 50Ω coaxial cableconnecting said baluns to said feed network.
 18. A transceive surfacecoil array according to claim 13 wherein said baluns are configured toprovide transmit signals generated by said feed network to said surfacecoils for transmit operation and to provide receive signals received bysaid surface coils to said feed network for receive operation.
 19. Atransceive surface coil array according to claim 18 wherein said feednetwork comprises transmit and receive components.
 20. A transceivesurface coil array according to claim 19 wherein said transmit andreceive components comprise 50Ω preamplifiers and low cost RF poweramplifiers.
 21. A transceive surface coil array according to claim 13wherein said baluns are configured to condition said surface coil arrayto a receive-only mode, in said receive-only mode, said baluns isolatingsaid surface coils from said feed network when said feed networkgenerates transmit signals.
 22. A transceive surface coil arrayaccording to claim 21 wherein said feed network comprises transmit andreceive components.
 23. A transceive surface coil array according toclaim 22 wherein said transmit and receive components comprise 50Ωpreamplifiers and low cost power amplifiers.
 24. A transceive surfacecoil array according to claim 4 wherein a magnetic decoupling circuitacts between each adjacent pair of surface coils.
 25. A transceivesurface coil array according to claim 4 wherein said surface coils arearranged in sets, magnetic decoupling circuits acting between adjacentpairs of surface coils in each set.
 26. A transceive surface coil arrayaccording to claim 11 wherein said feed network circuitry includes aseparate low power transmitter for each surface coil.
 27. A transceivesurface coil array according to claim 18 wherein said feed networkincludes a separate low power transmitter for each surface coil.
 28. Atransceive surface coil array according to claim 11 wherein receivesignals received by said surface coils bypass said feed networkcircuitry.
 29. A transceive surface coil array comprising: a surfacecoil support; a plurality of generally evenly spaced surface coilsmounted on said support and surrounding a volume into which a target tobe imaged is placed, each surface coil transmitting input transmitsignals and receiving incoming receive signals; and capacitivedecoupling circuitry acting between and magnetically decoupling saidsurface coils to inhibit magnetic coil-to-coil coupling, said capacitivedecoupling circuitry being electrically connected in parallel with saidsurface coils.
 30. A transceive surface coil array according to claim 29further comprising a feed network and impedance matching circuitryacting between said feed network and each of said surface coils.
 31. Atransceive surface coil array according to claim 30 wherein said feednetwork comprises transmit and receive components.
 32. A transceivesurface coil array according to claim 31 wherein said transmit andreceive components comprise 50Ω preamplifiers and low cost RF poweramplifiers.
 33. A transceive surface coil array according to claim 31wherein said impedance matching circuitry is configured to providetransmit signals generated by said feed network to said surface coilsfor transmit operation and to provide receive signals received by saidsurface coils to said feed network for receive operation.
 34. Atransceive surface coil array according to claim 33 wherein saidtransmit and receive components comprise 50Ω preamplifiers and low costRF power amplifiers.
 35. A transceive surface coil array according toclaim 30 wherein said impedance matching circuitry comprising a λ/4lattice balun associated with each surface coil.
 36. An RF resonatorcomprising: a surface coil array including an arrangement ofnon-overlapping magnetically decoupled surface coils encompassing avolume and a capacitive circuit between each pair of surface coils, eachcapacitive circuit being electrically connected in parallel with theassociated pair of surface coils, each surface coil transmitting inputtransmit signals and receiving incoming receive signals; and a feednetwork coupled to said surface coil array, said feed network receivingsignals received by said surface coils during imaging of a target withinsaid volume.
 37. An RF resonator according to claim 36 wherein said feednetwork comprises 50Ω transmit and receive components.
 38. An RFresonator according to claim 37 wherein said surface coil arraycomprising impedance matching circuitry acting between each surface coiland said feed network.
 39. An RF resonator according to claim 38 whereinsaid feed network provides transmit signals to said surface coils viasaid impedance matching circuitry.
 40. A transceive surface coil arraycomprising: a surface coil support; an arrangement of non-overlappingmagnetically decoupled surface coils mounted on said support andencompassing a volume into which a target to be imaged is placed, eachsurface coil transmitting input transmit signals and receiving incomingreceive signals; and a capacitive circuit between adjacent surfacecoils, each capacitive circuit being electrically connected in parallelwith the associated adjacent surface coils.
 41. A transceive surfacecoil array according to claim 40 wherein each of said surface coils isgenerally identical.
 42. A transceive surface coil array according toclaim 41 wherein said surface coils are of varying shape and/or size.43. A transceive surface coil array according to claim 42 wherein saidsurface coils are grouped into sets, the surface coils in each set beingmagnetically decoupled independent of other sets.
 44. A transceivesurface coil array according to claim 43 wherein said surface coilsreceive transmit signals from at least one radio frequency transmitter.45. A transceive surface coil array according to claim 44 wherein saidsurface coils receive transmit signals from a single transmitter.
 46. Atransceive surface coil array according to claim 45 wherein each surfacecoil receives transmit signals from an associated transmitter.